The invention relates to a four-wheel steering arrangement for motor vehicles having a first steering device for the front wheels operated by a steering wheel and having a second steering device for the rear wheels controlled by a hydraulic adjusting device as a function of determined driving condition quantities, fed to an electronic control computer unit to create a variable rear wheel steering angle ratio with respect to the front wheels.
On the basis of European Patent EP-A 0 096 345, a steering arrangement for the rear wheels of a motor vehicle in a four-wheel steering system is known and which comprises a hydraulic adjusting cylinder connected with a wheel suspension element and used for adjusting rear-wheel steering angles. These wheel adjustments, according to DE-OS 31 24 821, take place as a function of operating Parameters of the vehicle, which are processed in an electronic control unit and are fed as output control signals to a control device or multipath valve for controlling the adjusting cylinder.
The invention is based on the object of providing a steering arrangement for a four-wheel steering system for motor vehicles having a first steering device for the front wheels operated by a steering wheel and having a second steering device for the rear wheels controlled by a hydraulic adjusting device as a function of determined driving condition quantities, fed to an electronic control computer unit to create a variable rear wheel steering angle ratio with respect to the front wheels, which ensures an exact steering angle adjustment during the driving operation and an automatic restoring of the rear wheels into a straight-ahead position in the case of a failure of or a defect in the electronic and/or hydraulic control. The steering angles of the rear wheels must also remain unaffected by movements of the wheel suspension link, particularly springing movements. In addition, it should be possible, with very few changes, to retrofit a vehicle, particularly a vehicle having semi-trailing arms, with this steering device.
According to the invention, this object is achieved by having the steering device comprise a hydraulic control cylinder having a restoring device and both of which are connected by adjusting elements to the rear wheels for adjusting the rear wheel steering angles so that the wheels can be swivelled around an axis of rotation, which is formed by support joints between the wheel carrier and the wheel suspension link.
It is also advantageous if the adjusting elements consist of tie rods which are connected with the restoring device and the control cylinder, arranged transversely in the vehicle, and which are connected with longitudinally extending stationary steering levers of a wheel carrier by deflecting joints. Each deflecting joint is arranged between the tie rod and the steering lever along a swivel axis which extends through the bearing of the wheel suspension link at the vehicle body side. The deflecting joint is located adjacent to the swivel axis in such a manner that no change of toe-in occurs during springing movements of the vehicle.
It is also advantageous if the wheel suspension link consists of a semi-trailing arm device which comprises bearing arms which accommodate the wheel carrier between each arm by joints, one joint being arranged above a wheel axis and the other joint being arranged below the wheel axis and wherein these joints form the wheel rotating axis.
The spring restoring device comprises an adjusting rod which is connected with a piston rod of the control cylinder and with the tie rod and which has pressure springs provided to achieve forced restoring movements of the rear wheels into a straight-ahead position with a steering angle .beta.=0. The adjusting rod is guided in a cylinder tube. A stop disk is fixed to the rod and the pressure springs are arranged against movable pressure disks which can be shifted with respect to the stop disk, and are supported at a stationary sectional ring of the cylinder tube at opposite bearing bushes at the ends of the cylinder to hold the adjusting rod in a center position (M) by the stop disk.
According to an alternative embodiment, the adjusting rod is guided axially slidably between slidable pressure disks, in between which disks is a pressure spring. The Pressure disk support themselves at end stops of the tube to hold the adjusting rod in its position by exterior stop disks fixed at the rod.
For controlling, the control cylinder is coupled with a throttle valve and shutoff valves which are arranged in a hydraulic pressure circuit between a pressure supply unit and the adjusting cylinder. The throttle valve is connected in front of the control cylinder and is located in series with two shutoff valves. One shutoff valve is arranged in the feeding pipe of the cylinder while another shutoff valve is arranged in the return pipe. The throttle and shutoff valves provided for an automatic and relatively slow wheel restoring to a center position (M) when the power supply is interrupted. The throttle valve is spring biased to automatically take up a throttling position and the shutoff valves are spring biased into a shutoff position.
The main advantages achieved by the invention are that a steering device for the rear wheels of a motor vehicle is provided which consists of few components and can be installed in an uncomplicated manner in a vehicle, particularly a vehicle having a semi-trailing arm rear axle. It must be possible to convert in a simple manner a motor vehicle with the known semi-trailing arm axle to a vehicle with all-wheel steering, while its kinematic and elastokinematic characteristics are maintained.
By the special position of the site of the swivel connection between a steering tie rod and a steering lever, an unintentional influencing of the toe-in of the rear wheels is avoided during springing movements of the semi-trailing arm device. If a targeted change of the toe-in of the rear wheel is to be achieved by a wheel lift, this may be influenced in a simple manner by a changed position of the deflecting joint, for example, next to the swivelling axis between the tie rod and the steering lever, as well as by an inclining of the tie rod.
The control cylinder is coupled with a restoring device which affects the cylinder mechanically so that when the electric and/or hydraulic control system fails, the control cylinder is affected such that the rear wheels, which are subjected to a steering angle, are slowly, against the disturbing forces in the wheel tread point, restored to a straight-ahead position. In this position, they are held fast without play while the steered rear axle, in the unpressurized state, has the characteristics of an unsteered rear axle. For this purpose, a throttle valve is provided in the hydraulic control circuit of the steering system by which the pressure medium can flow from one chamber of the control cylinder into the other chamber. In the throttled position, the shutoff valves of the hydraulic steering circuit for the control cylinder are simultaneously also shut. In these positions of the valves, the control cylinder is fixed in its center position by the force of the restoring springs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.